Add targets to build doc-stubs for non-updatable

Like the jar stubs, the doc stubs need modularization to avoid depending
on module sources and instead being assembled by merging the doc stubs
of the constituent modules. This will allow generating e.g. the lint
database, annotation zips, and the documentation itself from prebuilts
(which in turn allows mixing e.g. non-updatable R with the latest
modules).

Move the defaults for the non-updatable jar stubs to the shared bp file
and reuse it for the non-updatable doc stubs, and add two targets for
the public and the system stubs.

Bug: 186197911
Test: m android-non-updatable-doc-stubs{,-system}
Change-Id: Ic2cef748c42c141ea42b8cd7b4afd7ee78a1399d